1. Field of the Invention
The present invention relates to horizontal hinged doors which are pivotally spring cylinder loaded toward the open position and, in particular, to a spring cylinder lifting mechanism whereby a portion of the spring forces necessary to open the door are distributed from the spring lifting mechanism structure to the door thus reducing the forces on the mechanism especially at the upper pivot point and enabling the lifting mechanism to be made with engineering materials having a wide range of strengths such as plastics.
2. Description of Related Art
Hatch or door structures are usually mounted horizontally or at least at an angle to the vertical, as in the case of roof scuttles for buildings. The structures are commonly termed horizontal doors and are typically used in sidewalks, in the floors of industrial facilities for access to subterranean locations and for roof access in large buildings. Doors of this type are often made of metal, typically steel or aluminum and due to their weight, they are normally counterbalanced for safety and ease of operation.
In general, the doors are made utilizing a frame, typically a channel frame, surrounding the door with the frame being secured to the door opening. The metal cover (door) for the doorway is hinged to the frame so that it swings upwardly to the open vertical position or downwardly to the closed horizontal position. The heavy weight of the metal cover necessitates the need for a counter balancing mechanism to aid in the opening and/or closing of the door. Typically, the counter balancing mechanism comprises two concentric cylinders mounted between the frame and the door and containing a spring therein. The spring cylinders comprise two cylindrical halves sealed at the ends, which slide together about an internal spring. As the door is closed, the spring inside the spring cylinders is compressed thereby providing a counter balancing force. Similarly, when the door is open the spring provides a force swinging the door upward to the open position. The bottom half of the spring cylinder is typically rotably (pivotally) connected to the frame at its lower end. The concentric upper half of the spring cylinder is rotably (pivotally) connected at its upper end to a flange on the underside of the door. Accordingly, when the door is moved to an open or closed position, the spring cylinders pivot about their upper and lower pivot axis either compressing or decompressing the spring.
In some cover constructions, the force of the lifting springs is insufficient to prevent the door from accidentally swinging closed and a locking means is typically provided to prop the door in its open position. Quite often a door will have a fixed ladder mounted below the door. When a user is descending the ladder and desires to close the overhead door, he must be able to do so with one hand while holding onto the ladder with the other hand. Accordingly, the props are designed so that the prop can be disengaged with one hand while closing the door as shown in U.S. Pat. No. 5,205,073 to Robert Lyons, Sr. and assigned to the assignee of the present invention. U.S. Pat. No. 5,205,073 is incorporated herein by reference.
Due to the weight of the doors, the springs used to counter balance the doors are very strong and produce large forces in the spring housings in both the open door and closed door positions. The forces generated by the spring act on each halve of the spring housings with the forces being generally concentrated at the pivot points at the frame flange and at the door flange. Metal has been the preferred material of construction for the spring cylinders but plastics are now becoming more desirable due to their increased strength and other operating characteristics such as corrosion resistance. Unfortunately, the plastics and other like materials typically do not have the strength of metal especially fatigue strength and since the doors are generally designed for a 50 year life the door may be opened and closed thousands of times during this time. The forces acting on the spring cylinders especially plastic cylinders will eventually cause fracture of the cylinders with the cylinder then needing replacement. The use of plastic and other inherently weak materials for the spring cylinder housings is thus complicated by the extreme spring forces as well as the opening and closing forces and the use of plastic spring cylinder housings in horizontal and other door assemblies is severely restricted because of the demands on the material.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a door assembly and in particular a horizontal hinged door assembly with counter balancing spring means wherein cylinders containing spring means used to open and close the doors are made of plastic or other non-metallic or metallic materials.
It is a further object of the present invention to provide spring cylinder housings for door assemblies including horizontal hinged door assemblies which housings are made of plastic or other non-metallic or metallic materials.
Other objects and advantages of the present invention will be readily apparent from the following description.